snapsvg

2014-10-01

What's wrong with JavaScript in the template?

Those of you keeping score will know that I recently started a new job. This one is Perl, not PHP, and so a certain level of standards is expected from the code. What with Perl having all these neato features and excellent web frameworks, I at least consider it on a par with Python and Ruby in its utility.

Perusing the new-to-me codebase I of course discover some of the hysterical raisins that live there, much of which is easily forgiven because the original coder had the foresight to apologise in a comment for doing it in the first place. But one thing stood out to me as a prime candidate for refactoring: JavaScript in the templates.

I said as much and was surprised to be posed the question, "What's wrong with JavaScript in the templates?"

Surprised not to be asked the question, but because I didn't know what the answer was. I've worked enough on the front end of previous jobs to have enough experience in the matter that seeing JS in template code makes me flinch, but never have I been asked to actually introspect this reaction and explain it.

Questions like that are primo blog post material, and it's been a while since I properly got my teeth into one, so on my journey home I put my mind to formalising quite what it was about it that made me want to rip it out and refactor the life out of it.

What it's not

Some obvious answers come to mind, with varying validity.

Is it because it's hard to find? No. Everything's hard to find. ack for it - you'll find it soon enough.

Is it because it violates separation of concerns? No. In fact, you could argue that it improves it, by encapsulating JavaScript only useful to a template inside that very template.

Is it because the only reason most people put JS in a template is so they can use the templating language to build JS? Well yes, but that's just the same question. What's wrong with it?

Is it because it's not reusable? Well, yes and no. Most template JS is not intended to be reusable; it's quite specific to that particular template, and there's little use for it elsewhere. More on this point later.

Is it the same reason we don't put CSS in the template either? Or inline in the HTML? Yes! By Jupiter, yes! We find the answer in the template itself. It's the other, main part of the template that we've not mentioned yet - the HTML.

What lies beneath

To answer the question, we must deconstruct the web page itself and look at the parts. What are we really looking at when we look at a web page? What are we really providing when we build a template? What is the purpose of the HTML, the TT2 or Jade or Mustache code that wraps or creates it?

Most web pages follow a similar structure: There's the <html> with its <head> and <body>; the body has a <div class="header"> or, better yet, a <header>, and some sort of <div id="content">. Then at last there's a bunch of stuff that finally gets to the point, i.e. displays whatever it is the page is displaying.

Most template structures separate all the pre/postamble from the content itself. Even in the CGI days we, naively but with good intent, would have a header.html and a footer.html and we would render the header, then the body, then the footer, to STDOUT. More recently, we have a single file with the pre- and postamble in it, and we import the rendered content into that. We tend to also have a considerable number of satellite template files representing handy widgets and reusable code and all the other things that I've alredy said aren't really the reason why we don't do the title of this article.

We knew then, as we know now, something we always forget to talk about; something implicit in everything we do here. While we make all these templates rendering data in consistent ways we somehow lose sight of the simplest of notions: we are representing resources.

Resource and Framing

"Resource" is a fully-functional word, writ deep into the very clay with which we make our internets; vis-a-vis HTTP. HTTP works with a verb and a noun, i.e. it says "Do this to this". "Framing" is a word I've picked to describe what it is we website-makers do to resources to make them look nice for people using browsers that conform to the standards set out to allow us to do so.

HTTP's nouns are URIs. URI means Uniform Resource Identifier. The R in URI (or URL or IRI) means resource. It means thing; it's identifying the nouns of the internet. We respond to a (request to a) URI with a resource, represented in HTML format for the purposes of this discussion. We know this, but we never say this - and so whenever we get discussions, no one ever uses it as a basis for finding answers. But the concept of resource contains the answer to our question.

When we divide our templates up into separate files there is the tacit goal that the template we use to represent the actual, specific resource contain as little HTML as possible. Why? Well, mostly for consistency. We want to frame all our resources - at least those related to each other - in the same way. That means that if we put as little HTML as we can get away with into our resource templates, we can put as much as we can get away with into our framing templates, and thus have as little variation between the rendered resources as we can. A side effect, and therefore a second benefit, is that if we want to reuse or amend our framing, we can do this in one place - it's DRY.

We already recognise the difference between frame and resource: it's encoded right there in <div id="content">. How many of your templates resemble this structure?

That right there is the boundary between Alliance and Reaver space. Uh, I mean, the place where the framing goes away and the resource begins. The resource is all the data that change when you ask for a different ID, or a different resource type. The resource is that which, if you took all the HTML away, would still be what you asked for.

I've nearly made my point

Not all resources are data. Some resources are forms. I'm choosing forms as an example for another resource type because we're all familiar with them doing stuff.

Forms contain no data, but instead prompt you for data, and allow you to create more resources. Nominally, they represent the structure of the resource type, but don't represent any particular record of that type. The form holds the key to the answer: behaviour.

This is a form with a file control, as you well know. It renders as a box with a "Browse" button. This one renders with a label, "Upload image:".

If you click on the label, the text of the input, or the browse button, you get the same behaviour: a file browser pops up. When you select a file and confirm it, the name of the file appears in the text part of the input, unless some jackass has installed Uploadify or similar, and broken it.

It also renders a single submit button. The button looks like all the other buttons on your website because you don't put CSS in your templates. The reason for that is being explained as we speak. I mean, as you read. I mean now.

When you click the submit button, the browser composes an HTTP POST request to the URL /upload_image on the host that served this resource. This request contains the entirety of the selected file, encoded in such a way that the receiving server can understand it. Presumably, the resource at that URL knows what to do with it.

Now, kindly point out to me the part of the HTML snippet above that implements any of that behaviour.

It's not there.

Nouns and adjectives - that's what the HTML is made of. There is not a single verb in the entirety of that form, and yet those few lines perform, implicitly, functionality that you would probably have to look up on Wikipedia to implement yourself.

Not all resources are forms, either. Here's a video resource, shamelessly stolen from Wikipedia, and represented in HTML format:

Fetches the resource at '/movie.jpg' of the host that served this HTML resource, and renders it at the place in the page concordant with the styling associated with it and the rest of the HTML.

Puts some sort of controls on this image, probably a play button, which, when clicked, causes the resource at '/movie.webm' to be fetched.

Renders the fetched video file in situ, replacing the still image, and plays any sound that comes with it.

Renders further controls, such as a scrubber, pause, volume slider.

Affects the right-click menu of the browser to provide appropriate options to a video: save video, get URL, get URL at this time, etc.

Plus anything else I've forgotten. The <img> noun has similar, albeit many fewer, effects: the image is fetched and rendered without user interaction. Indeed, if the image is an animated gif, it will animate! On its own!

This borderline-facetious set of examples serves to point out that the browser has already got verbs. The nouns (HTML elements) say which verbs you want to use (and where to put the visuals for the user's interaction), and the adjectives (the attributes of the elements) control the parameters that the verbs need. (Fetch which video? Play automatically?)

This is called semantics.

Semantics!

I'm going to define semantics as the use of nouns to imply verbs1. Form fields come with behaviour, and you say which behaviour you want through nouns, i.e. the choice of which input you use. Semantics also covers those adjectives that fine-tune the noun's behaviour by describing it further.

Semantics tell things how to behave based on what the resource contains. An HTML resource often contains framing. Semantics go into the HTML to tell anyone who cares which bit they can ignore. Semantics is the way you phrase things; it's how you describe the resource.

Consider:

<div id="content">

A web scraper can use this sort of thing to know what to ignore. Ignore is a verb. The HTML doesn't say "ignore this"; that's for the client to decide.

The browser isn't going to ignore it - but the browser doesn't care about this particular piece of semantics2. If the CSS says to do something to it then the browser will do that to it, but the browser doesn't do that by default.

The web scraper will skip anything outside this div - provided it knows what the 'content' ID means - and the browser will do nothing based on this ID because it hasn't been told to.

That right there is the answer. There is a difference between all the things it is possible for a browser to do and all the things the browser can already do. You can stick together awesome websites entirely using HTML5 and CSS3, but often you want behaviour that is not already built-in to the browser. Maybe you want div#content to have special styling or behaviour, but browsers don't come with that built-in.

And indeed, styling is just a form of behaviour - CSS tells the browser how to behave when it renders certain elements in certain configurations. JavaScript tells the browser how to behave when the user does things.

This is the point where people start putting JavaScript into templates. A specific form needs special behaviour, so you add a <script> tag and then output the form.

Smash! go the semantics. Fie! cry the tortured frontenders.

None of the behaviour you ever write is useful only once. I told you I'd get back to the reusability point. The JavaScript doesn't go in the template because it's not reusable, sure, but why is that a problem?

The problem is the JavaScript defines verbs. Semantic HTML is that HTML which uses only nouns, and lets the browser select the correct verbs.

JavaScript, therefore, is correctly a separate resource that adds verbs to the browser, and defines the nouns to which they apply. That's why everything eventually ends up as a JavaScript plugin; and sometimes as core browser behaviour.

Essentially, we're saying that JavaScript is a CSS file that defines behaviour, not styling. Where CSS tells the browser how to interpret the semantics of your HTML in terms of colouring, positioning and so on, JavaScript tells the browser how to interpret the semantics in terms of direct functionality - behaviour.

Indeed, not only should JavaScript never go into the template, it should never go into <script> tags either. Just like CSS should never go into <style> tags.

The Related Resource

Resources have related resources. If you strip out all the framing of your HTML resource (e.g. you render it as JSON instead) you are still going to keep many of the hyperlinks - the contents of any <a> tag inside the content div, perhaps some of the image sources. That's because the HTML framing is just rendering the content in a human-readable way3. The relations between resources are actually part of the resource itself, or at least metadata to it.

This is important because it addresses one of the main reasons people put JavaScript in templates: so that they can use the template language on the JavaScript, and thus build resource-specific JS that renders, e.g., a list of related resources when you click some "See related" button.

If the resources are related they should already be in the page. I seriously cannot stress that enough. Either the related resources are, or are not, relevant to this representation of the resource.

If the HTML went away and you were returning JSON, would you, or would you not, list those related resources as metadata, one way or another?

They cannot be part of the framing: the framing is consistent across the whole site! They are unique to this resource; and the style of list that is invisible until a button is pressed is unique to this type of resource.

But is "style of list" not an adjective about this list? Is list not a noun? Cannot you use the noun-adjective semantics to say, "This is a list of related resources, and it is of type pop-up-on-button"? HTML is amply equipped to represent this semantically: we even have the rel attribute to let you specify which button should activate the list.

Related resources belong in the page. Either as a hyperlink, or directly in the HTML. If you want to save bandwidth, you don't put the whole list in, but you put in a hyperlink placeholder instead. The important thing is that the HTML is accurately representing the resource. Just like the JSON would. Don't force non-browser consumers of your HTML resource to figure out how to run the JavaScript just to get related data.

e.g.

This|http://harvesthq.github.io/chosen/

is Chosen. You've probably seen it before. You start typing in a form field, and it lists all matching options, filtering as you type.

Chosen can either use an existing set of options, such as from a select box, or a URL from which to fetch options that match the string.

Both of these can be in the HTML before the JS even runs. The list of options is a related resource; it is simply represented in different ways. The first way puts all of the related resources in with the main resource; the second way puts a hyperlink to a single other related resource, from which they can be fetched when it's appropriate to do so.

At no time is it necessary to put this data into the JavaScript. JavaScript can read. Hell, the JavaScript should work on the JSON representation and all you'd have to change would be how it finds the data.

The Answer

The answer, then, is semantics. Of course it is. But it's what semantics means that turned out to be the difficult thing to define here.

Semantics is about saying what this resource is; it's metadata about the resource itself. Semantics allows the client to make the decisions about what parts of the resource are relevant and what parts are not.

It's exactly the same principle by which responsive web design works.

It's exactly the same reason you don't put inline CSS into your HTML.

It's exactly the same reason you've never written a video player, or had to decode the JPEG file format manually in JavaScript and blit the resulting bitstring onto a canvas element.

It's exactly the same reason you don't know how to launch a file browser dialogue box.4